1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive maintaining a substantially constant host transfer rate when reading data from varying data rate data tracks across multiple disk surfaces.
2. Description of the Prior Art
Prior art disk drives employ one or more disks with heads actuated over the respective disk surfaces (e.g., top and bottom surfaces). Each disk surface comprises a plurality of radially spaced, concentric data tracks, wherein each data track comprises a number of data sectors for storing user data. During write operations, the disk drive receives user data and a logical block address (LBA) which is mapped to an absolute block address (ABA) identifying one of the data sectors for storing the user data. The LBA to ABA mapping enables defective data sectors to be mapped to spare data sectors.
Because the disk is rotated at a constant velocity, the data rate is typically increased toward the outer diameter tracks (where the surface of the disk is spinning faster) in order to achieve a more constant linear bit density across the radius of the disk. To simplify design considerations, the data tracks are typically banded together into a number of physical zones, wherein the data rate is constant across a zone, and increased from the inner diameter zones to the outer diameter zones. This is illustrated in FIG. 1 which shows a prior art disk format 2 comprising a number of data tracks 4, wherein the data tracks are banded together in this example to form three physical zones from the inner diameter of the disk (ZONE 1) to the outer diameter of the disk (ZONE 3). The prior art disk format of FIG. 1 also comprises a number of embedded embedded servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts) which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
Prior art disk drives typically configure the physical zones so that the zone boundaries occur at the same radial location across all disk surfaces, and so the data rate of each zone is the same across the disk surfaces. This is understood with reference to FIG. 1 by viewing the physical zone boundaries as cylinders extending through the disk surfaces so that each disk surface comprises three physical zones. In addition, the data rate is the same in each physical zone across the disk surfaces, for example, the data rate of physical zone 1 is the same for each disk surface. When a serpentine pattern is used to access the disk surfaces while reading a sequence of consecutive data sectors, the host transfer rate remains substantially constant since the data tracks are recorded at the same data rate from surface to surface (as long as the serpentine pattern does not cross a physical zone boundary). This results in a substantially constant stream of data sectors when viewed from the host, for example, when the host is running a benchmark program for quality verification.
A more recent development in disk drives is to optimize the data rate of each physical zone as well as the size of each physical zone relative to the characteristics of each head/disk interface. The radial density (tracks-per-inch (TPI)) may also be optimized for each disk surface which can also cause the size of each physical zone (and the corresponding physical zone boundaries) to differ across the disk surfaces. As a result, the data rate from surface to surface will change when reading a consecutive sequence of data sectors in a serpentine pattern that extends across the disk surfaces. This can lead to a varying host transfer rate depending on the current disk surface being accessed. When viewed from a host running a benchmark program, the varying transfer rate may suggest the disk drive is defective, when in fact nothing is wrong.
There is, therefore, a need to maintain a substantially constant average host transfer rate in a disk drive employing varying data rate data tracks across multiple disk surfaces.